Monday November 1 2010
There are still uncertainties about what intensity of exercise is best
“Getting sweaty in your spare time means you’ll be twice as likely to stave off depression than someone who doesn’t,” the Daily Mirror has reported. However, the newspaper says that the link only exists when people get active for their own leisure, but not when doing a physically demanding job.
The news is based on a large study of Norwegian citizens, which found that higher levels of leisure-time activity (intense or light) were associated with reduced likelihood of depression, although work-based activity was not. There are some shortcomings to the research, which the researchers themselves discuss. But importantly, this study cannot prove the direction of the link, i.e. whether exercising more leads to less depressive symptoms or the equally plausible case that people who are more depressed are less likely to exercise.
The researchers also found that levels of social support and social engagements may partially explain this link, and should be researched as it may be a distinguishing feature between workplace and leisure activity. Overall, these results should be interpreted in terms of what is already known about exercise and mental health, and not in isolation.
Where did the story come from?
The study was carried out by researchers from King’s College London and from the University of Bergen. Grants to individual researchers were provided by the National Institute for Health Research’s Biomedical Research Centre for Mental Health, the Institute of Psychiatry, the Institute of Social Psychiatry and the Norwegian Research Council. The study was published in the peer-reviewed British Journal of Psychiatry.
Several newspapers have covered this research well, although it is important to highlight that the design of this study means that it cannot prove the direction of the link between exercise and health.
What kind of research was this?
In this large cross-sectional study of Norwegians, researchers tested whether greater physical activity was linked to a lower likelihood of common mental health problems. They were interested in calculating the size of the link in relation to both leisure-time physical activity (i.e. not relating to work) and working in a physically active environment.
Many studies have established the beneficial effects of exercise on mental health, but the researchers say that there are still uncertainties about the intensity of exercise that is best. In this study, they hoped to explore further the ‘dose-relationship’ between exercising and mental health benefits, i.e. to what degree increasing activity levels were linked to lower risks of mental health problems.
What did the research involve?
Between 1995 and June 1997, all inhabitants of a Norwegian county aged 20 to 89 years old (total of 92,936 people) were invited for a clinical examination. The researchers included only those who agreed to participate and had provided enough data to analyse. This amounted to 40,401 participants.
Their levels of physical activity were assessed by asking them how often they engaged in light and intense leisure-time physical activity. Light physical activity was defined as an activity that did not lead to being sweaty or out of breath. Intensive activity was defined as that which leads to breathlessness or sweating. The response options were ‘none’, ‘less than one hour a week’, ‘one to two hours a week’, or ‘more than three hours a week’. Participants were also asked how physically active they were at work and could respond ‘mostly sedentary’, ‘required to walk a lot’, ‘walk and lift a lot’, or ‘intense physical work’.
Depression and anxiety were assessed using a self-report scale called the Hospital Anxiety and Depression (HAD) scale, which asks about depression symptoms in the previous two weeks. Details about a range of factors that could confound the relationship between exercise and depression were also collected. These factors included age, gender, family history of mental illness, current social class, education, marriage status, cigarette use, alcohol problems, physical problems and impairment due to physical illness.
The researchers then compared the odds of depression, anxiety or both across the different exercise categories. They did this both before and after adjusting for different factors. They structured their analysis so that doing no exercise, less than an hour a week, and one to two hours a week would be compared with three hours a week. In a final step, they combined cases of depression and ‘comorbid depression’, i.e. depression in the presence of anxiety.
What were the basic results?
In the sample providing data, 10% had symptoms of depression and 15% had symptoms of anxiety. There was some overlap between the groups, with 5.6% of the total study population having symptoms of both anxiety and depression.
There was an inverse relationship between light and intense leisure-time physical activity and depression (both with and without comorbid anxiety), i.e. greater levels of activity were associated with a lower risk of depression. This relationship remained even after accounting for possible confounding factors.
There was no link between intense leisure activity and anxiety, although there was some evidence that light leisure activity reduced the likelihood of anxiety. Neither depression nor anxiety was linked with workplace activity.
The link between leisure-time activity and depression was reduced when the researchers adjusted for additional factors, including resting pulse, cholesterol levels, blood glucose, BMI, waist-to-hip ratio, how many good friends the participants had, and how often they engage in social activities.
How did the researchers interpret the results?
The researchers say that in a large community-based sample, they have noted an inverse association between leisure-time physical activity and depression, with symptoms of depression being more common in people who reported no leisure-time physical activity. They say they also found evidence that social factors such as social engagement and support may “partially explain this relationship”.
This was a large cross-sectional study that has provided more detail on the relationship between leisure-time and work-based physical activity. The study has several strengths, notably its size and the detailed information collected on both social and biological factors. The researchers highlight some of the study’s limitations:
- They relied on self-reported levels of activity.
- The HAD scale of mental health used in this study may misclassify some people who have a mental disorder with primarily physical symptoms. The scale cannot provide a clinical diagnosis of either depression or anxiety. However, studies have shown that the performance of the HAD scale is as good as other scales for detecting symptoms of depression and anxiety.
- The study had a cross-sectional design, i.e. assessed a range of factors at a single point in time. This means it cannot prove the direction of the link between exercise and depression, i.e. whether those who did more exercise are subsequently less likely to develop depressive symptoms, or the equally plausible case that people who are less depressed are more likely to exercise.
- The researchers note that the participants were drawn from a rural area where people are more likely to be active. Therefore it is not certain whether the results are representative of other populations.
- In such studies, it is important to take into account all possible factors that may be associated with exercise levels or mental health. The researchers say that it is possible that the analyses did not fully adjust for all important factors.
Importantly, this research (and subsequent news reports) should be interpreted in light of the main limitation of the researchers, who say they “are not able to make any firm conclusions on the direction of causation in any of the associations described”, i.e. that it is uncertain whether inactivity might cause depression or whether depression leads to inactivity. They add that “it is likely that there may be some reverse causation”.
However, the number of social engagements and social support that participants reported appeared to at least partially explain the link between leisure-time activity and reduced likelihood of depression. Therefore this social aspect of leisure time activity appears to be important.
In general, this study adds to the evidence that exercise is good for mental health, although in isolation it does not prove a causal link between the two. It should be discussed in the context of what else is known about the benefits of exercise.